1. Field of the Invention
The invention is a radiography apparatus to be used for computerized tomography. More specifically, this invention relates to an x-ray tube, beam collimation and data collection system to be used with a computerized tomograph in spiral scanning.
2. Description of the Prior Art
Computerized Tomography (CT) is a standard imaging technique in the radiology clinic. Spiral CT scanning is a technique, whereby the patient is transported through the x-ray beam during continuous rotation of the x-ray beam. The spiral technique results in a significant reduction in examination times versus the conventional CT slice-by-slice technique.
Trade-offs between volume coverage, scan time and image time quality are required in the selection of the spiral CT scan parameters-exposure technique factors, slice width, and patient table speed. In particular, the scanned volume may be increased with no increase in scan time through the use of faster table speed, however the faster speeds result in larger slice sensitivity profiles and reduced Z-dimension spatial resolution of the CT images.
While it is desirable to decrease the scan time for a given volume, or conversely increase the volume coverage for a given scan time through the use of faster table speeds, there is a reduction in image quality with the use of higher table speeds. The influence of table speed or pitch (slice width/table speed) on image quality has been documented in the literature (Polacin et al, Radiology 1992; 185:29-35). In particular, the use of pitch values greater than 1 results in severe broadening of the slice sensitivity profiles. The use of 180.degree. interpolation schemes serves to reduce the broadening of the slice sensitivity profiles at the cost of higher image noise. However, even with 180.degree. interpolation, there is still slice sensitivity profile broadening for large pitch values. For example, Polacin et al report a 30 percent slice sensitivity profile broadening with pitch=2 and a 180.degree. linear interpolation (LI).
Spiral CT examinations, due to their short scan times and reduced patient motion, produce superior multi-planar reformations (MPR) and 3D images. In addition, spiral CT has also enabled the development of CT angiography. The image quality of MPR, 3D and MIP (maximum intensity projection) images is dependent on the image quality of the axial CT slices. In particular, the Z-dimension spatial resolution of MPR, 3D and MIP images is dependent on the slice sensitivity profile of the axial CT images and the spacing of the axial CT images. Conflicting desires are encountered to obtain maximum scan volume coverage (i.e. maximum Z-dimension scan length) while simultaneously preserving high Z-dimension spatial resolution. In particular, spiral examinations with high pitch values enable large scan volume coverage, however the slice sensitivity profiles broadening of the axial CT slices reduces the Z-dimensional spatial resolution of the MPR, 3D and MIP images.
There remains, therefore, a need for method and apparatus to enable large volume acquisitions while preserving minimal scan times while simultaneously maintaining high Z-dimensional spatial resolution. In addition, there is a need for a method and apparatus to enable large volume acquisitions and high Z-dimension spatial resolution for MPR, 3D and MIP images.
There is further need for such a method and apparatus which is compatible with existing computerized tomography systems and is economical to manufacture and use.